Correctly update wl_notificationtimestamp when viewing old revisions

Prelude

wl_notificationtimestamp controls sending the user e-mail
notifications about changes to pages, as well as showing the "updated
since last visit" markers on history pages, recent changes and
watchlist.

The bug

Previously, on every view of a page, the notification timestamp was
cleared, regardless of whether the user as actually viewing the latest
revision. When viewing a diff, however, the timestamp was cleared only
if one of the revisions being compared was the latest one of its page.

The same behavior applied to talk page message indicators (which are
actually stored sepately to cater to anonymous users).

This was inconsistent and surprising when one was attempting to, say,
go through the 50 new posts to a discussion page in a peacemeal
fashion.

The fix

If the revision being viewed is the latest (or can't be determined),
the timestamp is cleared as previously, as this is necessary to
reenable e-mail notifications for given user and page.

If the revision isn't the latest, the timestamp is updated to
revision's timestamp plus one second. This uses up to two simple
(selectField) indexed queries per page view, only fired when we
do not already know we're looking at the latest version.

Talk page indicator is updated to point at the next revision after the
one being viewed, or cleared if viewing the latest revision. The
UserClearNewTalkNotification hook gained $oldid as the second argument
(a backwards-compatible change). In Skin, we no longer ignore the
indicator being present if we're viewing the talk page, as it might
still be valid.

The bonus

Comments and formatting was updated in a few places, including
tables.sql and Wiki.php.

The following functions gained a second, optional $oldid parameter
(holy indirection, Batman!):

• WikiPage#doViewUpdates()
• User#clearNotification()
• WatchedItem#resetNotificationTimestamp()

DifferenceEngine gained a public method mapDiffPrevNext() used
to parse the ids from URL parameters like oldid=12345&diff=prev,
factored out of loadRevisionIds(). A bug where the NewDifferenceEngine
hook would not be called in some cases, dating back to its
introduction in r45518, was fixed in the process.

Bug: 41759
Change-Id: I4144ba1987b8d7a7e8b24f4f067eedac2ae44459